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mm: mmu_notifier: have mmu_notifiers use a global SRCU so they may sa…
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…fely schedule

With an RCU based mmu_notifier implementation, any callout to
mmu_notifier_invalidate_range_{start,end}() or
mmu_notifier_invalidate_page() would not be allowed to call schedule()
as that could potentially allow a modification to the mmu_notifier
structure while it is currently being used.

Since srcu allocs 4 machine words per instance per cpu, we may end up
with memory exhaustion if we use srcu per mm.  So all mms share a global
srcu.  Note that during large mmu_notifier activity exit & unregister
paths might hang for longer periods, but it is tolerable for current
mmu_notifier clients.

Signed-off-by: Sagi Grimberg <[email protected]>
Signed-off-by: Andrea Arcangeli <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Haggai Eran <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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@torvalds
Sagi Grimberg authored and torvalds committed Oct 9, 2012
1 parent 48af0d7 commit 21a9273
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Showing 2 changed files with 49 additions and 25 deletions.
1 change: 1 addition & 0 deletions include/linux/mmu_notifier.h
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@
#include <linux/list.h>
#include <linux/spinlock.h>
#include <linux/mm_types.h>
#include <linux/srcu.h>

struct mmu_notifier;
struct mmu_notifier_ops;
Expand Down
73 changes: 48 additions & 25 deletions mm/mmu_notifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -14,32 +14,37 @@
#include <linux/export.h>
#include <linux/mm.h>
#include <linux/err.h>
#include <linux/srcu.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/slab.h>

/* global SRCU for all MMs */
struct srcu_struct srcu;

/*
* This function can't run concurrently against mmu_notifier_register
* because mm->mm_users > 0 during mmu_notifier_register and exit_mmap
* runs with mm_users == 0. Other tasks may still invoke mmu notifiers
* in parallel despite there being no task using this mm any more,
* through the vmas outside of the exit_mmap context, such as with
* vmtruncate. This serializes against mmu_notifier_unregister with
* the mmu_notifier_mm->lock in addition to RCU and it serializes
* against the other mmu notifiers with RCU. struct mmu_notifier_mm
* the mmu_notifier_mm->lock in addition to SRCU and it serializes
* against the other mmu notifiers with SRCU. struct mmu_notifier_mm
* can't go away from under us as exit_mmap holds an mm_count pin
* itself.
*/
void __mmu_notifier_release(struct mm_struct *mm)
{
struct mmu_notifier *mn;
struct hlist_node *n;
int id;

/*
* RCU here will block mmu_notifier_unregister until
* ->release returns.
*/
rcu_read_lock();
id = srcu_read_lock(&srcu);
hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist)
/*
* if ->release runs before mmu_notifier_unregister it
Expand All @@ -50,7 +55,7 @@ void __mmu_notifier_release(struct mm_struct *mm)
*/
if (mn->ops->release)
mn->ops->release(mn, mm);
rcu_read_unlock();
srcu_read_unlock(&srcu, id);

spin_lock(&mm->mmu_notifier_mm->lock);
while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) {
Expand All @@ -68,15 +73,15 @@ void __mmu_notifier_release(struct mm_struct *mm)
spin_unlock(&mm->mmu_notifier_mm->lock);

/*
* synchronize_rcu here prevents mmu_notifier_release to
* synchronize_srcu here prevents mmu_notifier_release to
* return to exit_mmap (which would proceed freeing all pages
* in the mm) until the ->release method returns, if it was
* invoked by mmu_notifier_unregister.
*
* The mmu_notifier_mm can't go away from under us because one
* mm_count is hold by exit_mmap.
*/
synchronize_rcu();
synchronize_srcu(&srcu);
}

/*
Expand All @@ -89,14 +94,14 @@ int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
{
struct mmu_notifier *mn;
struct hlist_node *n;
int young = 0;
int young = 0, id;

rcu_read_lock();
id = srcu_read_lock(&srcu);
hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
if (mn->ops->clear_flush_young)
young |= mn->ops->clear_flush_young(mn, mm, address);
}
rcu_read_unlock();
srcu_read_unlock(&srcu, id);

return young;
}
Expand All @@ -106,17 +111,17 @@ int __mmu_notifier_test_young(struct mm_struct *mm,
{
struct mmu_notifier *mn;
struct hlist_node *n;
int young = 0;
int young = 0, id;

rcu_read_lock();
id = srcu_read_lock(&srcu);
hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
if (mn->ops->test_young) {
young = mn->ops->test_young(mn, mm, address);
if (young)
break;
}
}
rcu_read_unlock();
srcu_read_unlock(&srcu, id);

return young;
}
Expand All @@ -126,8 +131,9 @@ void __mmu_notifier_change_pte(struct mm_struct *mm, unsigned long address,
{
struct mmu_notifier *mn;
struct hlist_node *n;
int id;

rcu_read_lock();
id = srcu_read_lock(&srcu);
hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
if (mn->ops->change_pte)
mn->ops->change_pte(mn, mm, address, pte);
Expand All @@ -138,49 +144,52 @@ void __mmu_notifier_change_pte(struct mm_struct *mm, unsigned long address,
else if (mn->ops->invalidate_page)
mn->ops->invalidate_page(mn, mm, address);
}
rcu_read_unlock();
srcu_read_unlock(&srcu, id);
}

void __mmu_notifier_invalidate_page(struct mm_struct *mm,
unsigned long address)
{
struct mmu_notifier *mn;
struct hlist_node *n;
int id;

rcu_read_lock();
id = srcu_read_lock(&srcu);
hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
if (mn->ops->invalidate_page)
mn->ops->invalidate_page(mn, mm, address);
}
rcu_read_unlock();
srcu_read_unlock(&srcu, id);
}

void __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
unsigned long start, unsigned long end)
{
struct mmu_notifier *mn;
struct hlist_node *n;
int id;

rcu_read_lock();
id = srcu_read_lock(&srcu);
hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
if (mn->ops->invalidate_range_start)
mn->ops->invalidate_range_start(mn, mm, start, end);
}
rcu_read_unlock();
srcu_read_unlock(&srcu, id);
}

void __mmu_notifier_invalidate_range_end(struct mm_struct *mm,
unsigned long start, unsigned long end)
{
struct mmu_notifier *mn;
struct hlist_node *n;
int id;

rcu_read_lock();
id = srcu_read_lock(&srcu);
hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
if (mn->ops->invalidate_range_end)
mn->ops->invalidate_range_end(mn, mm, start, end);
}
rcu_read_unlock();
srcu_read_unlock(&srcu, id);
}

static int do_mmu_notifier_register(struct mmu_notifier *mn,
Expand All @@ -192,6 +201,12 @@ static int do_mmu_notifier_register(struct mmu_notifier *mn,

BUG_ON(atomic_read(&mm->mm_users) <= 0);

/*
* Verify that mmu_notifier_init() already run and the global srcu is
* initialized.
*/
BUG_ON(!srcu.per_cpu_ref);

ret = -ENOMEM;
mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
if (unlikely(!mmu_notifier_mm))
Expand Down Expand Up @@ -274,8 +289,8 @@ void __mmu_notifier_mm_destroy(struct mm_struct *mm)
/*
* This releases the mm_count pin automatically and frees the mm
* structure if it was the last user of it. It serializes against
* running mmu notifiers with RCU and against mmu_notifier_unregister
* with the unregister lock + RCU. All sptes must be dropped before
* running mmu notifiers with SRCU and against mmu_notifier_unregister
* with the unregister lock + SRCU. All sptes must be dropped before
* calling mmu_notifier_unregister. ->release or any other notifier
* method may be invoked concurrently with mmu_notifier_unregister,
* and only after mmu_notifier_unregister returned we're guaranteed
Expand All @@ -290,16 +305,17 @@ void mmu_notifier_unregister(struct mmu_notifier *mn, struct mm_struct *mm)
* RCU here will force exit_mmap to wait ->release to finish
* before freeing the pages.
*/
rcu_read_lock();
int id;

id = srcu_read_lock(&srcu);
/*
* exit_mmap will block in mmu_notifier_release to
* guarantee ->release is called before freeing the
* pages.
*/
if (mn->ops->release)
mn->ops->release(mn, mm);
rcu_read_unlock();
srcu_read_unlock(&srcu, id);

spin_lock(&mm->mmu_notifier_mm->lock);
hlist_del_rcu(&mn->hlist);
Expand All @@ -310,10 +326,17 @@ void mmu_notifier_unregister(struct mmu_notifier *mn, struct mm_struct *mm)
* Wait any running method to finish, of course including
* ->release if it was run by mmu_notifier_relase instead of us.
*/
synchronize_rcu();
synchronize_srcu(&srcu);

BUG_ON(atomic_read(&mm->mm_count) <= 0);

mmdrop(mm);
}
EXPORT_SYMBOL_GPL(mmu_notifier_unregister);

static int __init mmu_notifier_init(void)
{
return init_srcu_struct(&srcu);
}

module_init(mmu_notifier_init);

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